Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1405 (proposed Site JA-14A; 40°08.30′N, 51°49.20′W; 4287 m water depth) (Fig. F1) is a mid- to deepwater site located on J-Anomaly Ridge that was planned to capture a record of sedimentation 657 m shallower than the largely sub–carbonate compensation depth (CCD) record drilled at Site U1403 (Figs. F2,F3,F4). Our scientific objectives for drilling Site U1405 were

To test the hypothesis that this seismically homogeneous package is a plastered drift deposit of Paleogene age,

To obtain a sedimentary section suited to paleoceanographic study at suborbital timescales featuring very high rates of deposition compared to those typically found in deep sea pelagic sites (1–2 cm/k.y.), and

Coring at Site U1404 recovered an ~300 m thick sequence of clays and oozes of Miocene, Oligocene, and late middle to late Eocene age featuring distinct increases in calcium carbonate content in sediment of Eocene, Eocene–Oligocene transition (EOT), and late Oligocene–early Miocene age. Prior to drilling, Site U1405 was hypothesized to be more carbonate rich than Site U1404 in time-equivalent strata and also a sensitive recorder of those CCD shoaling and deepening events not quite large enough to have triggered changes in calcium carbonate accumulation at Site U1404. Initial results from Site U1403 indicated that the CCD in the North Atlantic Ocean during the early Eocene was much deeper (by ~1.5 km) than in the contemporaneous equatorial Pacific Ocean (Pälike et al., 2012), whereas initial results from Site U1404 indicated transient carbonate pulses and inferred CCD deepening events during the middle to late Eocene and EOT time intervals that bear some similarities to those seen in the Pacific Ocean. Coring at Site U1404 also recovered evidence of a pulse of carbonate deposition during the early Miocene and latest Oligocene that is not recognized in the tropical Pacific (Pälike et al., 2012). At Site U1405, our aim was to obtain a sedimentary record of one or more of these intervals of geologic time in shallower water than at Site U1404 and in a more expanded section to provide a more detailed paleoceanographic record of these events.

For the most part, average linear sediment accumulation rates documented at Site U1404 are characteristic of normal pelagic sedimentation (≤1.5 cm/k.y.), but sedimentation rates in the latest Oligocene and early Miocene are noticeably higher (~7 cm/k.y.) and closer to rates anticipated in sediment drift deposits. Site U1405 was positioned to penetrate the acoustically uniform sediment package at close to its maximum thickness (Fig. F4) to determine the extent to which these seismically distinctive features, common across J-Anomaly and Southeast Newfoundland Ridges, were in fact sediment drift deposits. Sediment drifts of Pliocene–Pleistocene age have been drilled in the northeast Atlantic and are shown to feature high sedimentation rates (4–20 cm/k.y.) suited to the study rates of abrupt climate change (e.g., Channell et al., 2010). At Site U1405, we expected to encounter a sediment sequence similar to that encountered at Site U1404 in terms of its overall stratigraphy, but we did not know whether upslope thickening between the two sites was attributable to higher long-term rates of carbonate burial in shallower water depths (a CCD effect) or to higher rates of clay or silica delivery to the site during geologically short intervals of time (drift sedimentation).

Finally, drilling at Site U1405 was also needed to help understand the history of chemical stratification and ocean currents in the Paleogene North Atlantic Ocean. Most deep-ocean drill sites are located at mid-ocean depths in the Paleogene when we account for thermal subsidence. Hence, Site U1405 is rare in that it was selected to recover a representative sequence deposited in true deep water (~4000 meters below sea level [mbsl] in the Eocene). Furthermore, the site is well placed to record the history of deep water formed in the far North Atlantic, or even the Arctic, because the Deep Western Boundary Current is constrained to flow directly over or around the Newfoundland ridges by geostrophic flow and the shape of the ocean basin. At the deep end of the J-Anomaly Ridge depth transect, the site should record the chemistry and flow history of abyssal waters in the Paleogene.